Thanks, Bill, that definitely works (see the loop below), however, I think
it's not really effective from the CPU resources consumption standpoint.
Maybe it has to be done at the kernel level...
char res[2];
....
// check for value 0 i.e. 0x30
while (res[0] != 0x30) {
lseek(fd, 0, SEEK_SET);
read(fd, res, 2);
}
On Friday, October 12, 2018 at 4:37:52 PM UTC-6, Bill Fleming wrote:
>
> I'm pretty sure that you can just always read the "value" inside the GPIO
> tree using a file read command.
> So for example if you try to set the pin a certain way and it didn't
> change (strong pull or short) the value read still reflects the actual
> state.
>
> So just write a C program using fopen and fread to read the value from the
> "file".
> http://www.cplusplus.com/reference/cstdio/fread/
>
> Put the read function in its own separate thread in a loop with a task
> delay resolution to match the baud rate.
>
> Then you are done, assuming the output pin read does actually work that
> way.
> If not you can try setting input mode, read the value, then set output
> mode right after you are done reading.
>
> Bill
>
> On Friday, October 12, 2018 at 11:06:36 AM UTC-5, Sergey Manucharian wrote:
>>
>> What's the best way to detect a change in value on a GPIO configured as
>> OUT?
>> It may sound really weird why one may need that. I have a custom cape and
>> cannot make changes to its hardware, but I need to improve certain
>> functionality.
>> poll(2) on /sys/class/gpio/gpioXY/value exits with timeout.
>>
>> To avoid any XY-problem I'll explain the actual situation:
>> The BBB's native RS-485 mode works fine with omap_serial driver, however,
>> I cannot use actual hardware pins for RTS (RE/DE) because the above
>> mentioned cape consumes all possibilities.
>> So, I assigned an unconnected GPIO to be the RTS and want to detect its
>> state changes in a timing-critical application (tcdrain() takes too long).
>>
>> Thanks for advises!
>> Sergey
>>
>>
--
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